1. Field of the Invention
The present invention relates to a switch arrangement and, more particularly, to a time delay switching device which includes at least two switches and a pneumatic timer triggerable by an electromagnetic means, with each of the at least two switches having a movable switching element in order to enable a control of operating sequences in, for example, wye-delta circuits.
2. Prior Art
Time delay switching devices actuated by electromagnetic means have been proposed, usually in the form of accessories, wherein switches of the switching device switch directly and jointly after a delay period has elapsed, with only the distances which the individual bridges of the switches must travel producing a slight difference in the switching time and, in actual practice, the break contacts assume an open position a few milliseconds before the make contacts close.
Switching devices of the aforementioned type are used extensively in sequential controls which require a time delay switching operation. In certain applications such as, for example, in star or wye-delta circuits, the nearly simultaneous switching moment of the break contacts and make contacts which switch over after a delay time has elaspsed lead to certain technical problems. As a rule, despite the generally conventional electrical interlocking, the pause in the switching operation between a wye operation and delta operation is insufficient to extinguish the arc which appears at the contacts of the wye switch before the contacts of the delta switch have contacted another. In, for example, electric motors, a disadvantage of the above noted electrical interlocking resides in the fact that arc-air shorts result as well as the generation of magnetic residual fields.
In order to avoid the above noted disadvantages, mechanical time delay relays especially designed for wye-delta contactors have been proposed, with the relays including switches provided with a switching pause between the opening of the break contacts and closing of the make contacts. Additionally, electronic time delay relays have also been proposed for wye-delta contactors which, for the reasons noted above, employ two output relays. A disadvantage of employing two completely indendently operating time delay relays resides in the fact that a relatively high cost is incurred and, moreover, the provision of a number of different voltage ranges are necessary to adapt the control voltage of the wye-delta contactors.